It is the aim of the proposed project to develop a quantitative and qualitative assay for human hemopoietic pluripotent stem cells in mice. There is a great need for a qualitative and quantitative stem cell assay in man, to evaluate differences between normal and leukemic stem cells, to monitor different kinds of hematologic disorders and chemotherapy of many forms of cancer, to determine the nature of defects in cases of anemia and immune deficiency diseases. Several laboratories have attempted this important goal, but to date failed. Recently, we were able to abrogate xenogeneic resistance to rat bone marrow grafts in strongly incompatible combination by several pretreatments of prospective recipient mice. We believe that failure of other investigators was due to xenogeneic resistance to hemopoietic grafts. In an attempt to promote the growth of human hemopoietic cells in mice two main approaches will be used: 1) mice of certain genetic constitutions (called "nonresponders" or "poor responders" to hemopoietic grafts) as well as genetically immunodeficient mice (asplenic and athymic-asplenic); 2) drugs and agents known to abrogate xenogeneic resistance such as cytotoxic drug cyclophosphamide, powerful adjuvant Corynebacterium parvum, anti-lymphocyte globulin, antimacrophage agents. All recipient mice will be lethally irradiated prior to infusion of donor cells. Human marrow and peripheral blood cell (normal and leukemic) will be used as a source of donor cells.